I. Field of the Invention
This invention relates generally to electrocardiograph apparatus and/or programming equipment for cardiac rhythm management devices (CRMD) employed to monitor and record cardiac depolarization and repolarization signals, and more particularly to the incorporation into such equipment of a minute ventilation detector for providing an indication of when an implanted MV (minute ventilation-based) rate adaptive CRMD is generating a AC carrier signal used in deriving a minute ventilation related control signal for the implanted device.
II. Discussion of the Prior Art
One type of rate adaptive cardiac pacemaker on the market today uses a patient""s minute ventilation as a rate control parameter. In such rate adaptive pacemakers/defibrillators, transthoracic impedance is measured by applying an AC carrier signal of a predetermined frequency, typically about 30 KHz, from an oscillator between an electrode carried by a pacing lead disposed in the heart and an electrode on the pacemaker can that is usually located in a surgically formed pocket beneath the pectoral muscle in the patient""s chest. This AC carrier signal is modulated by respiratory activity (inspiration and expiration) and a rate control signal is derived by demodulating the carrier. The Hauck et al. U.S. Pat. No. 5,318,597 may be referred to for additional disclosure of the construction and mode of operation of MV based rate adaptive pacemakers.
While the frequency of the AC carrier is generally outside of the bandwidth of most physiologic signals, the amplitude of the carrier frequently dominates electrocardiograph input signals and can interfere with the detection of pacing pulses put out by the implanted device. It is also true that electrocardiograph equipment, especially those incorporating leads-off indication, can adversely affect operation of an implanted minute ventilation-based rate adaptive pacemaker, causing erroneously high pacing rates.
The need, therefore, exists for a way to better render electrocardiograph and patient programmers used with CRMDs compatible with minute ventilation-based rate adaptive pacemakers.
A solution to the foregoing undesired interaction resides in providing an ecg recorder for detecting and displaying cardiac signals picked up on a plurality of skin-contacting surface electrodes disposed on a patient at predetermined body locations in whom a rate-adaptive cardiac rhythm management device is implanted with a MV detector. The cardiac rhythm management device can be of a type that may have minute ventilation as a control parameter for the rate at which the device produces pacing pulses. The minute ventilation based rate adaptive device includes a means for impressing a sub-threshold carrier signal of a given high frequency in timed bursts of a predetermined repetition rate between a first electrode disposed within a patient""s thoracic cavity and a reference electrode. The MV detection circuit in the ecg recorder or CRMD programmer is connected to receive signals picked up by pairs of the plurality of surface electrodes, such signals including cardiac signal components, pacing pulse components, noise components and components due to the attenuating current carrier. The minute ventilation detection circuit indicates the time of occurrence of the components due to the carrier signal.
Without limitation, the minute ventilation detection circuit may comprise a matched filter that is coupled to receive the signals picked up by the pairs of surface electrodes along with a template comprising signals of the given high frequency, or, if in the digital domain, approximate filter coefficients. A comparator is coupled to receive the output of the matched filter and a reference signal. The comparator produces an output indicating the time of occurrence of the components due to the carrier signal when the output of the matched filter exceeds the reference signal.